copier

Repository: jinzhu/copier: Copier for golang, copy value from struct to struct and more (github.com)

Documentation: jinzhu/copier: Copier for golang, copy value from struct to struct and more (github.com)

copier is a library for type copying in Go, mostly used for struct-to-struct conversion. It has the same author as gorm and features the following:

• Deep copy
• Copy fields with the same name
• Copy slices
• Copy maps
• Copy methods

Since copier's copying relies on reflection, there will be some performance loss. Generally, this type of type conversion library is divided into two categories: one based on reflection, like copier, and the other based on code generation, which generates type conversion code. This method does not cause performance loss, and a similar library is jmattheis/goverter.

Installation

go get github.com/jinzhu/copier

Usage

This library is very simple to use but very practical. It only exposes two functions, one is copier.Copy.

func Copy(toValue interface{}, fromValue interface{}) (err error)

The other is copier.CopyWithOption, which allows some custom configuration for the copy behavior. By default, deep copy is not performed.

type Option struct {
  IgnoreEmpty   bool
  CaseSensitive bool
  DeepCopy      bool
  FieldNameMapping []FieldNameMapping
}

func CopyWithOption(toValue interface{}, fromValue interface{}, opt Option) (err error)

Here's an example of converting between different struct types. The User and Student structs are two completely different types with no association.

type User struct {
  Id   string
  Name string
  // Ignore this field when used as the destination struct
  Address string `copier:"-"`
}

type Student struct {
  // Specify field name
  StudentId   string `copier:"Id"`
  StudentName string `copier:"Name"`
  Address     string
  School      string
  Class       string
}

func main() {
  student := Student{
    StudentId:   "123",
    StudentName: "jack",
    Address:     "usa",
    School:      "MIT",
    Class:       "AI",
  }
  user := User{}
  if err := copier.Copy(&user, &student); err != nil {
    panic(err)
  }
  fmt.Printf("%+v\n", student)
  fmt.Printf("%+v\n", user)
}

Output

{StudentId:123 StudentName:jack Address:usa School:MIT Class:AI}
{Id:123 Name:jack Address:}

Here's copying slices

func main() {
  student := []Student{
    {
      StudentId:   "123",
      StudentName: "jack",
      Address:     "usa",
      School:      "MIT",
      Class:       "AI",
    },
    {
      StudentId:   "123",
      StudentName: "jack",
      Address:     "usa",
      School:      "MIT",
      Class:       "AI",
    },
  }

  var user []User
  if err := copier.Copy(&user, &student); err != nil {
    panic(err)
  }
  fmt.Printf("%+v\n", student)
  fmt.Printf("%+v\n", user)
}

Output

[{StudentId:123 StudentName:jack Address:usa School:MIT Class:AI} {StudentId:123 StudentName:jack Address:usa School:MIT Class:AI}]
[{Id:123 Name:jack Address:} {Id:123 Name:jack Address:}]

Copying maps

type User struct {
  Id   string
  Name string
  // Ignore this field when used as the destination struct
  Address string `copier:"-"`
}

type Student struct {
  // Specify field name
  StudentId   string `copier:"Id"`
  StudentName string `copier:"Name"`
  Address     string
  School      string
  Class       string
}

func main() {
  student := Student{
    StudentId:   "123",
    StudentName: "jack",
    Address:     "usa",
    School:      "MIT",
    Class:       "AI",
  }

  src := make(map[string]Student)
  src["a"] = student
  src["b"] = student

  dest := make(map[string]User)

  if err := copier.Copy(&dest, &src); err != nil {
    panic(err)
  }
  fmt.Printf("%+v\n", src)
  fmt.Printf("%+v\n", dest)
}

Output

map[a:{StudentId:123 StudentName:jack Address:usa School:MIT Class:AI} b:{StudentId:123 StudentName:jack Address:usa School:MIT Class:AI}]
map[a:{Id:123 Name:jack Address:} b:{Id:123 Name:jack Address:}]

Customization

You can also customize the conversion method by passing in copier.TypeConverter

type TypeConverter struct {
  SrcType interface{}
  DstType interface{}
  Fn      func(src interface{}) (dst interface{}, err error)
}

As shown below

type User struct {
  Id   string
  Name string
  // Ignore this field when used as the destination struct
  Address string `copier:"-"`
}

type Student struct {
  // Specify field name
  StudentId   string `copier:"Id"`
  StudentName string `copier:"Name"`
  Address     string
  School      string
  Class       string
}

func main() {
  student := Student{
    StudentId:   "123",
    StudentName: "jack",
    Address:     "usa",
    School:      "MIT",
    Class:       "AI",
  }

  src := make(map[string]Student)
  src["a"] = student
  src["b"] = student

  dest := make(map[string]User)

  if err := copier.CopyWithOption(&dest, &src, copier.Option{
    IgnoreEmpty:   false,
    CaseSensitive: false,
    DeepCopy:      false,
    Converters: []copier.TypeConverter{
      {
        SrcType: Student{},
        DstType: User{},
        Fn: func(src interface{}) (dst interface{}, err error) {
          s, ok := src.(Student)
          if !ok {
            return User{}, errors.New("error type")
          }
          return User{
            Id: s.StudentId,
          }, nil
        },
      },
    },
    FieldNameMapping: nil,
  }); err != nil {
    panic(err)
  }
  fmt.Printf("%+v\n", src)
  fmt.Printf("%+v\n", dest)
}

Output

map[a:{StudentId:123 StudentName:jack Address:usa School:MIT Class:AI} b:{StudentId:123 StudentName:jack Address:usa School:MIT Class:AI}]
map[a:{Id:123 Name: Address:} b:{Id:123 Name: Address:}]